What graubär means, probably, is that slide film itself - the product - has a higher density range than negative film. The higher dynamic range of the negative film is actually "compressed" in a shorter density range on film. That makes scanning easier (any scanner has a dynamic range sufficient to record the entire density range of a colour negative, not many scanners have the density range sufficient to record the entire density range of a slide frame) but than one must, so to speak, "uncompress" again this density range.
Also, the nature of the scanning process is such that scanning of a negative frame shows the noise mostly in the highlight region (where it is more disturbing) while scanning of slides shows the noise mostly in the shadows region, where it is less disturbing. It is also my experience that scans of slides look much more natural and convincing than scans of negatives, on screen.
White balancing of scans from slides is also much simpler than white balancing of scans from negatives. White balancing of a scan from a negative can be very (very) tricky if one doesn't use a proper colour-managed workflow. Negative film in theory should give higher colour precision but that is true only when a proper colour-managed workflow is used, both in an analogue workflow and in an hybrid workflow.